Lead-modified zinc oxide voltage variable resistor

ABSTRACT

A VOLTAGE VARIABLE RESISTOR COMPOSITION CONSISTING ESSENTIALLY OF ZINC OXIDE AND, AS AN ADDITIVE, LEAD OXIDE. THE LEAD-MODIFIED ZINC OXIDE VOLTAGE VARIABLE RESISTOR HAS IMPROVED VOLTAGE NON-LINEAR PROPERTIES DUE TO THE FURTHER ADDITION OF BISMUTH OXIDE, CALCUIM OXIDE AND COBALT OXIDE.

MICHIO MATSUOKA ETAL Filed Oct. 16, 1969 Jan. 4, 1972 LEAD-MODIFIED ZINC OXIDE VOLTAGE VARIABLE RESISTOR INVENTORS MICHIO MATSUOKA TAKESHI MASUYAMA YO SHJO IIDA add/14! ATTOPT-"EY',

United States Patent 3,632,528 LEAD-MODIFIED ZINC OXIDE VOLTAGE VARIABLE RESISTOR Michio Matsuoka, Takeshi Masuyama, and Yoshio Iida,

Osaka-fu, Japan, assignors to Matsushita Electric Industrial Co., Ltd., Kadoma, Osaka, Japan Filed Oct. 16, 1969, Ser. No. 866,821 Claims priority, application Japan, Oct. 22, 1968, 43/77,736 Int. Cl. H01b 1/06 US. Cl. 252-518 6 Claims ABSTRACT OF THE DISCLOSURE A voltage variable resistor ceramic composition consisting essentially of zinc oxide and, as an additive, lead oxide. The lead-modified zinc oxide voltage variable resistor has improved voltage non-linear properties due to the further addition of bismuth oxide, calcium oxide and cobalt oxide.

The invention relates to compositions of voltage variable resistor ceramics having non-ohmic resistance and more particularly to compositions of varistors comprising zinc oxide having non-ohmic resistance due to the bulk thereof. t

Various voltage variable resistors such as silicon carbide varistors, selenium rectifiers and germanium or silicon p-n junction diodes have been widely used for stabilization of voltage or current of electrical circuits. The electrical characteristics of such a voltage variable resistor are expressed by the relation:

V n (u) where V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numerical value greater than 1. The voltage of n is calculated by the following equation:

where V and V are the voltages at given currents I and I respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the extent to which the resistors depart from ohmic characteristics.

In conventional varistors comprising germanium or silicon pn junction diodes, it is difficult to control the C-value over a Wide range because the voltage variable property of these varistors is not attributed to the bulk but to the p-n junction. On the other hand, silicon carbide varistors have voltage variable properties due to the contacts among the individual grains of silicon carbide bonded together by a ceramic binding material, and the C-value is controlled by changing a dimension in a direction in which the current flows through the varistors. Silicon carbide varistors, however, have a relatively low n-value and are prepared only for the purpose of obtaining a lower C-value.

An object of the present invention is to provide a composition of a voltage variable resistor having non-ohmic properties due to the bulk thereof and having a controllable C-value.

Another object of the present invention is to provide a 3,632,528 Patented Jan. 4, 1972 composition of a voltage variable-resistor characterized by a high n-value.

These and other objects of the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the single figure is a partly cross-sectional View of a voltage variable resistor according to the invention.

Before proceeding with a detailed description of the voltage variable resistors contemplated by the invention, their construction will be described with reference to the aforesaid figure of drawing wherein reference character 10 designates, as a whole, a voltage variable resistor comprising, as its active element, a sintered body having a pair of electrodes 2 and 3 applied to opposite surfaces thereof. Said sintered body 1 is prepared in a manner hereinafter set forth and is in any [form such as circular, square or rectangular plate form. Wire leads 5 and 6 are attached conductively to the electrodes 2 and 3, respectively, by a connection means 4 such as solder or the like.

A voltage variable resistor according to the invention comprises a sintered body of a composition consisting essentially of 90.0 to 99.95 mole percent of zinc oxide and 0.05 to 10.0 mole percent of lead oxide. Such a voltage variable resistor has non-ohmic resistance due to the bulk itself. Therefore, its C-value can be changed without impairing the n-value by changing the distance between said opposite surfaces. The shorter distance results in the lower C-value.

The higher n-value can be obtained when said sintered body consisting essentially of 97.0 to 99.9 mole percent of zinc oxide and 0.1 to 3.0 mole percent of lead oxide in accordance with the invention.

According to the present invention, the C-value can be lowered without changing the dimension or lowering n-value when said sintered body is of a composition consisting essentially of 82.0 to 99.9 mole percent of zinc oxide, 0.05 to 10.0 mole percent lead oxide and 0.05 to 8.0 mole percent of bismuth oxide.

A combination of a low C-value and a high n-value can be obtained .when said sintered body consists essentially of 94.0 to 99.8 mole percent of zinc oxide, 0.1 to 3.0 mole percent of lead oxide and 0.1 to 3.0 mole percent of bismuth oxide.

According to the present invention, the stability for ambient temperature and the electric load life test can be improved when said sintered body consists essentially of 82.0 to 99.9 mole percent of zinc oxide, 0.05 to 10.0 mole percent of lead oxide and 0.05 to 8.0 mole percent of calcium oxide.

Further, the stability for ambient temperature and the electric load life test is extremely improved when said sintered body consists essentially of 94.0 to 99.8 mole percent of zinc oxide, 0.1 to 3.0 mole percent of lead oxide and 0.1 to 3.0 mole percent of calcium oxide.

According to the present invention, the n-value is elevated when said sintered body consists essentially of 82.0 to 99.9 mole percent of zinc oxide, 0.05 to 10.0 mole percent of lead oxide and 0.05 to 8.0 mole percent of cobalt oxide.

The n-value is further elevated when said sintered body is of a composition consisting essentially of 94.0 to 99.8 mole percent of zinc oxide, 0.1 to 3.0 mole percent of lead oxide and 0.1 to 3.0 mole percent of cobalt oxide.

According to the present invention, a combination of high n-value and low C-value can be obtained when said sintered body is of a composition consisting essentially of 74.0 to 99.85 mole percent of zinc oxide, 0.05 to 10.0 mole percent of lead oxide, 0.05 to 8.0 mole percent of 3 cobalt oxide and 0.05 to 8.0 mole percent of bismuth oxide.

Further, the C-value is lowered and the n-value is extremely elevated when said sintered body is of a composition consisting essentially of 91.0 to 99.7 mole percent of zinc oxide, 0.1 to 3.0 mole percent of lead oxide, 0.1 to 3.0 mole percent of cobalt oxide and 0.1 to 3.0 mole percent of bismuth oxide.

According to the present invention, a combination of a high n-value, a low C-value and a high stability can be obtained when said sintered body is of a composition consisting essentially of 74.0 to 99.85 mole percent of zinc oxide, 0.05 to 10.0 mole percent of lead oxide, 0.05 to 8.0 mole percent of bismuth oxide and 0.5 to 8.0 mole percent of calcium oxide.

Further, a combination of an extremely high n-value, a low C-value and a high stability can be obtained when said sintered body is of a composition consisting essentially of 91.0 to 99.7 mole percent of zinc oxide, 0.1 to 3.0 mole percent of lead oxide, 0.1 to 3.0 mole percent of bismuth oxide and 0.1 to 3.0 mole percent of calcium oxide.

The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials of the compositions described in the foregoing description are mixed in a wet mill so as to produce homogeneous mixtures. The mixtures are dried and pressed in a mold into desired shapes at a pressure from 100 kg./cm. to 1000 kg./cm. The pressed bodies are sintered in air at a given temperature for 1 to 3 hours, and then furnace-cooled to room temperature (about 15 to about 30 C.).

The available sintering temperature is determined in view of electrical resistivity, non-linearity and stability and ranges from 1000 to 1450 C.

The pressed bodies are preferably sintered in nonoxidizing atmosphere such as nitrogen and argon when it is desired to reduce the electrical resistivity.

The mixtures can be preliminarily calcined at 700 to 1000 C. and pulverized for easy fabrication in the subsequent pressing step. The mixture to be pressed can be admixed with a suitable binder such as water, polyvinyl alcohol, etc.

It is advantageous that the sintered body be lapped at the opposite surfaces by abrasive powder such as sili con carbide in a particle size of 300 meshes to 1500 meshes.

The sintered bodies are provided, at the opposite surface thereof, with electrodes in any available and suitable method such as electroplating method, vacuum evaporation method, metallizing method by spraying or silver painting method.

The voltage variable properties are not practically affected by the kinds of electrodes used, but are affected by the thickness of the sintered bodies. Particularly, the C- value varies in proportion to the thickness of the sintered bodies, while the n-value is almost independent of the thickness. This surely means that the voltage variable property is due to the bulk of the body, but not to the electrode.

Lead wires can be attached to the electrodes in a per se conventional manner by using conventional solder having a low melting point. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent in order to connect the lead wires to the electrodes.

Voltage variable resistors according to this invention have a high stability to temperature and in the load life test, which is carried out at 70 C. at a rating power for 500 hours. The n-value and C-value do not change re markably after heating cycles and load life test. It is advantageous for achievement of a high stability to humidity that the resultant voltage variable resistors are embedded in a humidity proof resin such as epoxy resin and phenol resin in a per se Well known manner.

Presently preferred illustrative embodiments of the invention are as follows.

4 EXAMPLE 1 A mixture of zinc oxide and lead oxide in a composition of Table 1 is mixed in a wet mill for 3 hours. The mixture is dried and then calcined at 700 C. for 1 hour. The calcined mixture is pulverized by the motor-driven ceramic mortar for 30 minutes and then pressed in a mold into a shape of 17.5 mm. in diameter and 2.5 mm. in thickness at a pressure of 500 kg./cm.

The pressed body is sintered in air at 1350 C. for 1 hour, and then furnace-cooled to room temperature (about 15 to about 30 C.) The sintered disc is lapped at the opposite surfaces thereof by silicon carbide in a particle size of 600 meshes. Resulting sintered disc has a size of 14 mm. in diameter and 1.5 mm. in thickness. The silver paint electrodes commercially available are attached to the opposite surfaces of sintered disc by painting. The lead wires are attached to the silver electrodes by soldering. The electric characteristics of the resultant resistors are shown in Table 1. It will be readily understood that the zinc oxide sintered body incorporated with lead oxide in an amount of 0.05 to 10.0 mole percent is available for the voltage variable resistor. Particularly, the addition of lead oxide in an amount of 0.1 to 3.0 mole percent makes the voltage nonlinear property more excellent.

TABLE 1 PbO (mol. 0 (at PbO (mol 0 (at percent) 1 ma.) '11 percent) 1 ma.) n

EXAMPLE 2 Starting materials composed of 99.5 mole percent of zinc oxide and 0.5 mole percent of lead oxide are mixed, dried, calcined and pulverized in the same manner as those of Example 1. The pulverized mixture is pressed in a mold into a disc 17.5 mm. in diameter and 5 mm. in thickness at a pressure of 5 00 kg./cm.

The pressed body is sintered in air at 1350" C. for 1 hour, and then furnace-cooled to room temperature. The sintered disc is ground at the opposite surfaces thereof into the thickness shown in Table 2 by silicon carbide in a particle size of 600 meshes. The ground disc is provided with the electrodes and lead Wires at the opposite surfaces in a similar manner to that of Example 1. The electric characteristics of the resultant resistors are shown in Table 2, which shows the C-value varies approximately in proportion to the thickness of the sintered discs while the n-value is essentially independent of the thickness. It will be readily realized that the voltage nonlinear property of the resistors are attributed to the sintered body itself.

TABLE 2 C (at Thickness (111111.) 1 ma.) 1;.

EXAMPLE 3 C (at 1 B1203 (mol percent) TABLE 5 000 (mol percent) EXAMPLE 6 TABLE 6 C00 (mol percent) PbO (mol percent) Zinc oxide containing the additions of Table 6 is is fabricated into voltage variable resistors by the same fabrication process as Example 1. The electrical properties of the resulting resistors are shown in Table. 6. It will be easily realized that the combination of lead oxide, cobalt oxide and bismuth oxide as additives results in a remarkably excellent n-value and at the same time in a lower C-value.

PbO (mol percent) C (at 1 ma.)

TABLE 3 BizOa (mol percent) EXAMPLE 4 Zinc oxide incorporated with lead oxide and calcium oxide in the composition of Table 4 is fabricated into voltage variable resistors by the same process as that of PbO (mol percent) Example 1. The resulting resistors are tested according to the methods used in the electronic components parts. The load life test is carried out at 70 C. ambient temperature at 0.5 watt rating power for 500 hours. The heating cycle test is carried out by repeating 5 times the cycle in which said resistors are kept at 85 C. ambient temperature for minutes, cooled rapidly to -2-0 C. and then kept at such temperature for 3-0 minutes. Table 4 shows a diiterence in the C-value and n-value between resistors before and after the load life test. -It can be 30 readily realized that the combination of lead oxide and calcium oxide as additive is effective for electrical and environmental stability.

. 0 0 MIN CaO (mol percent) 0.0000. .llll 00 01 AC (percent) EXAMPLE 7 Zinc oxide containing the additions of Table 7 is fabricated into voltage variable resistors by the same 0 Ru A: A: 32197500128552.1925 aezccleaeaaaaarnar process as that of Example 1. The resulting resistors are tested under the same conditions as those of Example 4. Table 7 shows the initial C-values and the difi'erences in C-values and n-values after the load life test. It can Fr be easily realized that the initial C-value of the resistor is lowered and at the same time the stability for the electrical and environmental load tests is excellent by using the combination of lead oxide, bismuth oxide and calcium as an additive.

TABLE 7 Load life test Heating cycle test 131203 08.0

(mol (mol 0 (at A0 (per- An (per- Ac (per- An (perpercent) percent) lma.) cent) cent) cent) cent) PbO (mol percent) 4.0299380029924108 RTQZAQEZRW-LA A QwA QWQmALQO 5 5 11 oe mo na .4 00 00 0 What is claimed is:

1. A voltage variable resistor ceramic composition consisting essentially of zinc oxide and 0.05 to 10.0 mole percent of lead oxide.

2. A voltage variable resistor ceramic composition as claimed in claim 1 wherein said composition consists essentially of zinc oxide and 0.1 to 3.0 mole percent of lead oxide.

3. A voltage variable resistor ceramic composition as claimed in claim 1, wherein said composition further includes 0.05 to 8.0 mole percent of one oxide selected from the group consisting of bismuth oxide, calcium oxide and cobalt oxide.

4. A voltage variable resistor ceramic composition as claimed in claim 2 wherein said composition further includes 0.1 to 3.0 mole percent of one oxide selected from the group consisting of bismuth oxide, calcium oxide and cobalt oxide.

5. A voltage variable resistor ceramic composition as References Cited UNITED STATES PATENTS 6/1959 Schusterius 252-520 6/1970 Bowman 252518 DOUGLAS I. DRUMMOND, Primary Examiner U.S. Cl. X.R. 25252l 

